Most low-level CFG functions update loops automatically. The following
functions handle some more complicated cases of CFG manipulations:

remove_path: Removes an edge and all blocks it dominates.

split_loop_exit_edge: Splits exit edge of the loop,
ensuring that PHI node arguments remain in the loop (this ensures that
loop-closed SSA form is preserved). Only useful on GIMPLE.

Finally, there are some higher-level loop transformations implemented.
While some of them are written so that they should work on non-innermost
loops, they are mostly untested in that case, and at the moment, they
are only reliable for the innermost loops:

create_iv: Creates a new induction variable. Only works on
GIMPLE. standard_iv_increment_position can be used to find a
suitable place for the iv increment.

duplicate_loop_to_header_edge,
tree_duplicate_loop_to_header_edge: These functions (on RTL and
on GIMPLE) duplicate the body of the loop prescribed number of times on
one of the edges entering loop header, thus performing either loop
unrolling or loop peeling. can_duplicate_loop_p
(can_unroll_loop_p on GIMPLE) must be true for the duplicated
loop.

loop_version, tree_ssa_loop_version: These function
create a copy of a loop, and a branch before them that selects one of
them depending on the prescribed condition. This is useful for
optimizations that need to verify some assumptions in runtime (one of
the copies of the loop is usually left unchanged, while the other one is
transformed in some way).

tree_unroll_loop: Unrolls the loop, including peeling the
extra iterations to make the number of iterations divisible by unroll
factor, updating the exit condition, and removing the exits that now
cannot be taken. Works only on GIMPLE.